The creation of a hydrocarbon producing well can be broadly classified in three stages. The first stage includes the drilling of the well borehole, where it is desirable to measure properties of earth formations penetrated by the borehole and to steer the direction of the borehole while drilling. The second stage includes testing of formations penetrated by the borehole to determine hydrocarbon content and producability. The third stage includes monitoring and controlling production typically throughout the life of the well. Operations in all stages typically employ a downhole assembly that contains one or more sensors responsive to stage related drilling, formation, or production parameters of interest. Response data from the one or more sensors are telemetered to the surface of the earth and received by a second transceiver for processing and interpretation. Conversely, it is desirable to transmit data via the surface transceiver to the borehole transceiver to control stage related drilling, testing or production operations.
In many of the stage operations discussed above, it is not operationally feasible to use a “hard wire” communication link, such as one or more electrical or fiber optic conductors, between the borehole transceiver and the surface transceiver. When hard wire communication links are not feasible, electromagnetic (EM) telemetry systems offer one means for communicating between borehole and surface transceivers. Data transmission rates using EM communication links are typically much lower than those of hard wire communication links. Signal attenuation in EM communication links is typically much higher than that in hard wire communication links, for a given operational depth within a borehole.
As mentioned above, direct or hard wire communication links for data telemetry are often operationally impractical in many well stage operations. This is especially true in the borehole drilling stage, where measures of parameters of formations penetrated by the borehole are of interest. Systems for measuring such geophysical and other parameters within the vicinity of a well borehole typically fall within two categories. The first category includes systems that measure parameters after the borehole has been drilled. These systems include wireline logging, tubing conveyed logging, slick line logging, production logging, permanent downhole sensing devices and other techniques known in the art. Memory type or hard wire communication links are typically used in these systems. The second category includes systems that measure formation and borehole parameters while the borehole is being drilled. These systems include measurements of drilling and borehole specific parameters commonly known as “measurement-while-drilling” (MWD), measurements of parameters of earth formation penetrated by the borehole commonly known as “logging-while-drilling” (LWD), and measurements of seismic related properties known as “seismic-while-drilling” or (SWD). For brevity, systems that measure parameters of interest while the borehole is being drilled will be referred to collectively in this disclosure as “MWD” systems. Within the scope of this disclosure, it should be understood that MWD systems also include logging-while-drilling and seismic-while-drilling systems.
A MWD system typically comprises a downhole assembly operationally attached to a downhole end of a drill string. The downhole assembly typically includes at least one sensor for measuring at least one parameter of interest, control and power elements for operating the sensor, and a borehole transceiver for transmitting sensor response to the surface of the earth for processing and analysis. The downhole assembly is terminated at the lower end with a drill bit. A rotary drilling rig is operationally attached to an upper end of the drill string. The action of the drilling rig rotates the drill string and downhole assembly thereby advancing the borehole by the action of the rotating drill bit. A surface transceiver is positioned remote from the downhole assembly and typically in the immediate vicinity of the drilling rig. The surface transceiver receives telemetered data from the downhole transceiver. Received data are typically processed using surface equipment, and one or more parameters of interest are recorded as a function of depth within the well borehole thereby providing a “log” of the one or more parameters. Hard wire communication links between the borehole and surface transceivers are operationally difficult because the downhole assembly containing the borehole transceiver is rotated typically by the drill string.
In the absence of a hard wire link, several techniques can be used as a communication link for the telemetry system. These systems include drilling fluid pressure modulation or “mud pulse” systems, acoustic systems, and electromagnetic systems.
Using a mud pulse system, a downhole transmitter induces pressure pulses or other pressure modulations within the drilling fluid used in drilling the borehole. The modulations are indicative of data of interest, such as response of a sensor within the downhole assembly. These modulations are subsequently measured typically at the surface of the earth using a receiver means, and data of interest is extracted from the modulation measurements. Data transmission rates are low using mud pulse systems. Furthermore, the signal to noise ratio is typically small and signal attenuation is large, especially for relatively deep boreholes.
A downhole transmitter of an acoustic telemetry induces amplitude and frequency modulated acoustic signals within the drill string. The signals are indicative of data of interest. These modulated signals are measured typically at the surface of the earth by an acoustic receiver means, and data of interest are extracted from the measurements. Once again, data transmission rate, the signal to noise ratio of the telemetry system is small, and signal attenuation as a function of depth within the borehole is large.
Electromagnetic telemetry systems can employ a variety of techniques. Using one technique, electromagnetic signals are modulated to reflect data of interest. These signals are transmitted from a downhole EM transceiver, through intervening earth formation, and detected using a surface transceiver that is typically located at or near the surface of the earth. Data of interest are extracted from the detected signal. Using another electromagnetic technique, a downhole transceiver creates a current within the drill string, and the current travels along the drill string. This current is typically created by imposing a voltage across a non-conducting section in the downhole assembly. The current is modulated to reflect data of interest. A voltage between the drilling rig and a remote ground is generated by the current and is measured by a transceiver, which is at the surface of the earth. The voltage is usually between a wire attached to the drilling rig or casing at the surface and a wire that leads to a grounded connection remote from the rig. Again, data of interest are extracted from the measured voltage. When data are sent from the surface transceiver to the downhole transceiver, voltage is applied between a point on the rig and a remote ground. This, in turn, creates a current that travels along the drill string and casing, and is detected by the downhole transceiver in the form of a voltage across the non-conducting section of the downhole assembly.